Unfortunately at 19, they are legally an adult and there really isn't much that you can do to prevent them from leaving the country if they want to. If you are concerned that they might be getting into a bad situation, you can try calmly talking to them about the dangers of meeting someone that they only know online - especially if they are going to be in a country where the laws might be a lot different than they are in the US.
If you are the parent, I wouldn't necessarily outright forbid it, as that generally leads kids doing things out of spite, but try to talk to them calmly and rationally about the potential dangers, and see if they are willing to see your side of it.
My sister is 15 & met some weird guy from turkey, she is determined to get out of the country to meet him. They met on omegle & have a skips relationship. It's so weird. He's like 20. I think he's a pervert, he blocked my whole family & stuff on fb & skype so we can't communicate. Unfortunately you can't do anything. (I'm 13) when they are 19 they are grown to make their own decisions. If it's a middle east country: tell them about all the bombings/murder/crime/wars there. Maybe you can scare them out of it. Pull up a Internet clip of of. Tell them all of the cons of it. They may not be able to get back into the u.s. once they leave. Good luck:)